Investigators have determined that Southern California Edison power lines ignited the 2017 Thomas fire, a massive blaze in Ventura and Santa Barbara counties that killed two people and later gave rise to a massive mudflow that resulted in at least 21 deaths.

Following a 15-month probe by Ventura County Fire Department investigators, along with teams from Cal Fire and the U.S. Forest Service, officials found the fire was started by two power lines that slapped together during high winds on the evening of Dec. 4, 2017.

“A high wind event caused the power lines to come into contact with each other, creating an electrical arc,” the Ventura County Fire Department said in a press statement Wednesday. “The electrical arc deposited hot, burning or molten material onto the ground, in a receptive fuel bed, causing the fire. The common term for this situation is called ‘line slap,’ and the power line in question is owned by Southern California Edison.”